i had the same thoughts as you. A few tips with hindsight-
Don't major in kines/exphys. I was pondering it and took quite a few classes. A previous poster was right, they are basically glorified PE classes - not a lot of hard science.
Do major in bio if you're interested. Don't get discouraged by all the basic science at the start. It will seem like it has nothing to do with medicine/the human body/running, but it will all tie together by the time you get to upper level classes.
DO explore another interest you might have. minor in film, anthroplogy, english, fine arts, poly sci, etc etc. This is my one regret. I know at 18 all you're interested in is running but things change when you get older and you may want to change directions at some point. This makes it a lot easier. e.g - I wanted to go to med school and treat runners (as a lot of freshman runners do). I've been injured ofr a whilesince graduating college and still want to go into medicine but want to get into international aid/public health and medical journalism. Having some experience outside bio and chem would have mede the transition a lot easier.
Hope this helps.